Ingvaeonic nasal spirant law

In historical linguistics, the Ingvaeonic nasal spirant law (also called the Anglo-Frisian or North Sea Germanic nasal spirant law) is a description of a phonological development that occurred in the Ingvaeonic dialects of the West Germanic languages. This includes Old English, Old Frisian, and Old Saxon, and to a small degree Old Dutch (Old Low Franconian).

The sound change affected sequences of vowel + nasal consonant + fricative consonant. ("Spirant" is an older term for "fricative".) The sequences in question are -ns-, -mf-, and -nþ-, preceded by any vowel. The nasal consonant disappeared, sometimes causing nasalization and compensatory lengthening of the vowel before it. The nasalization disappeared relatively soon after in many dialects along the coast, but it was retained long enough to prevent Anglo-Frisian brightening of /ɑː/ to /æː/. The resulting long nasalized vowel /ɑ̃ː/ was rounded to /oː/ in most languages under various circumstances.
In Old Saxon on the other hand, the nasal consonant is later restored in all but a small handful of relic words, so that Old Saxon /fĩːf/ ('five') appears as /fiːf/ in all Middle Low German dialects, while Old Saxon /mũːθ/ ('mouth') appears as /mʊnd/ in all Middle Low German dialects. The Old Saxon words /ɣɑ̃ːs/ ('goose') and /ũːs/ ('us') appear variably with and without a restored consonant, an example being the combination of /ɣoːs/ and /ʊns/ on the Baltic coast.

...
Wikipedia